The present invention relates to a closure mechanism of the type operable alternately for discharging molten metal from a molten metal containing vessel and for blocking such discharge, the closure mechanism being of the type including a conical discharge nozzle and a shielding tube fitted over a portion of the exterior of the discharge nozzle.
In molten metal containing vessels, such as casting ladles and tundishes, a molten metal, for example a steel melt, may be discharged through a closure mechanism, for example a stopper lock or a sliding gate closure unit, into an intermediate vessel or to a continuous casting mold assembly. Closure mechanisms of this type include a lower discharge nozzle, for example attached to a sliding plate of a sliding plate closure unit. To this discharge nozzle is attached a protective tube or shielding tube for the purpose, inter alia, of preventing the environmental atmosphere from oxidizing the discharged molten metal. Conventionally there is provided a conical snug fit or joint between the discharge nozzle and the shielding tube to ensure a reliable seal and to prevent the entrance of surrounding air. Nevertheless, unavoidably some air enters into the interior of the shielding tube along such conical joint. The result is that some of the molten metal passing through the shielding tube becomes oxidized, thereby deteriorating the degree of purity of the molten metal, for example steel.
In the past, various proposals have been advanced to attempt to overcome this problem. Thus, it has been attempted to place gas permeable inserts in the shielding tube beneath the discharge nozzle and to supply a shielding gas, such as an inert gas, to such insert. French Pat. No. 1,474,632 discloses a device in which a snug fit between the discharge nozzle and the protective tube is not required, and rather wherein an open boundary zone therebetween is filled with a protective or shielding gas. This solution however is complex structurally and requires a considerable consumption of the protective gas.